Bug Description

Evaluate the following from the REPL:

* (let ((x 0)) (defun foo () (incf x)))
* (describe 'foo)

After doing this, the following error appears:

debugger invoked on a SB-INT:BUG in thread #<THREAD "initial thread" RUNNING {1002900F21}>:
Don't know how to use a DEBUG-SOURCE without a namestring or a form.
This is probably a bug in SBCL itself. (Alternatively, SBCL might have been
corrupted by bad user code, e.g. by an undefined Lisp operation like
(FMAKUNBOUND 'COMPILE), or by stray pointers from alien code or from unsafe
Lisp code; or there might be a bug in the OS or hardware that SBCL is running
on.) If it seems to be a bug in SBCL itself, the maintainers would like to
know about it. Bug reports are welcome on the SBCL mailing lists, which you
can find at <http://sbcl.sourceforge.net/>.

Type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
0: [ABORT] Exit debugger, returning to top level.

(SB-INT:BUG
"Don't know how to use a DEBUG-SOURCE without ~ a namestring or a form.")
0] ^C

The bug happens on my development machine with 1.0.50, but I also reproduced it on the latest version from the source repository as of 2011-08-11.